Mouse and rat genomes contain a class of 5-6 Kb sequences termed VL30. These sequences are present in 20-50 related copies and exhibit structural and biological properties reminiscent of both endogenous retroviruses and eukaryotic transposable elements such as the copia, 412, and 297 sequences in Drosophila. VL30 sequences encode a 5-6 Kb poly(A)containing RNA which is enhanced in a wide variety of spontaneous and chemically-transformed cells and recent data from our laboratory has shown that VL30-related sequences are also present in elevated amounts in non-transformed cells cultured in the presence of epidermal growth factor. Recent experiments also suggest that distantly related organisms including the human contain related sequences present in one or only a few copies per genome. This research will further address the biological significance of VL30 sequences by 1) more detailed analysis of the relationship between VL30 expression, mitogen stimulation, and cell transformation, 2) measurements of evolutionary sequence conservation and structure, and 3) an analysis of the variation in copy number and chromosomal location of VL30 sequences in a variety of laboratory strains and subspecies of M. musculus. These experiments will provide further insight into the origin and function of a class of DNA sequences with properties intermediate between transposable elements and replication competent retroviruses and whose expression correlates with the alterated growth characteristics of at least some transformed cells.